Flue gas denitrification

In flue gas denitrification (also called DeNOx), nitrogen monoxide (NO) and nitrogen oxides (NO _x ) are removed from the exhaust gas of coal or gas turbine power plants , for example , with the help of so-called primary or secondary measures .

In the primary measures, the thermal NO formation is to be suppressed as far as possible through optimized combustion processes. The secondary measures, on the other hand, are separation processes in which the nitrogen oxides (NO _x ) contained in the flue gas are to be reduced by absorption by spraying in a scrubbing solution or by reduction to elemental nitrogen , for example by spraying in ammonia .

In the case of reductive processes, a distinction is made between selective non- catalytic reduction ( SNCR) and selective catalytic reduction (SCR).

Primary denitrification

The formation of NO _x in the furnace is favored by four factors:

high flame or combustion temperature . The combustion temperature can be recorded using the acoustic gas temperature measurement.
large supply of oxygen
long residence time of the reacting substances in the flame area
high pressures, such as those found in internal combustion engines, also promote NO _x formation.

Under certain conditions, nitrogen oxides are formed because the air components oxygen O ₂ (volume fraction 20%) and nitrogen N ₂ react directly with one another (e.g. in an electric arc furnace ).

Primary denitrification is characterized by the fact that it reduces these factors through the appropriate construction of burners and appropriate combustion concepts. These include:

Flue gas recirculation
Air staging
Fuel staging

Flue gas recirculation

By injecting or recirculating flue gas into the combustion zone, on the one hand the proportion of oxygen is reduced and on the other hand the combustion temperature drops. The combustion temperature depends, among other things, on the calorific value of the fuel. Fuel and combustion air together form a combustible mixture with a characteristic calorific value (earlier , today ). If flue gas is added to the fuel-oxidizer mixture, the calorific value of the total mixture and thus the attainable combustion temperature are reduced. ${\ displaystyle H_ {u}}$ ${\ displaystyle H_ {i}}$

This process is one of the most variable concepts in the industry. This measure is carried out by so-called “low NO _x burners”.

Flue gas recirculation is a measure to reduce the generation of thermal NO _x and is therefore not very effective with fuels in which the proportion of fuel NO _x in the exhaust gas dominates.

In internal combustion engines, the same process is known as exhaust gas recirculation .

Air staging

In this concept (English "air staging") the intention is to create several combustion zones around the flame that have different oxygen concentrations that increase from the inside out. This increases the area of combustion and thus the dwell time of the components in the flame. The step air can be added either in the burner itself or in the area of the combustion chamber.

The aim of the concept is to reduce the formation of thermal NO _x through the fuel-rich “primary zone” and to carry out the complete combustion of the fuel in the oxygen-rich and cooler “secondary zone” in stages.

Fuel grading

With this concept (also known as “fuel staging”), the fuel is also fed into the combustion chamber in several (usually two) stages. Similar to the air staging, this can be used to create zones with different fuel-air ratios, which also bring about a reduction in NO _x emissions. In the melting chamber firing system , up to 15% of the heat output of the steam generator is generated simply by adding and igniting fuel gas into the exhaust gas flow. The remaining 85% is generated in the actual melting chamber with a slight excess of air . The O ₂ not consumed here is used to burn the fuel gas. An analogy of this procedure can be found in the afterburner of the flight engines.

literature

Erich Fitzer, Dieter Siegel: Nitrogen oxide emissions from industrial combustion systems as a function of the operating conditions. In: Chemical Engineer Technology . No. 47 (13), 1975, ISSN 0009-286X , p. 571.
Hans-Georg Schäfer, Fred N. Riedel: About the formation of nitrogen oxides in large combustion plants, their influence on the environment, their reduction and their removal from the exhaust gases of the power plants. In: Chemiker-Zeitung. No. 113 (2), 1989, ISSN 0009-2894 , pp. 65-72.
Manfred Köbel, Martin Elsener: Denitrification of exhaust gases using the SNCR process: ammonia or urea as a reducing agent? In: Chemical Engineer Technology. No. 64 (10), 1992, ISSN 0009-286X , pp. 934-937.